A terminal in an optical communication network comprises a plurality of known components, such as waveguide-optical directional couplers, photodetectors, waveguide branches and laser diodes. These components are described in detail in the literature. For example, an article in IEEE Spectrum, May 1984, pages 28-33, by S. Kobayashi and T. Kimura entitled "Semiconductor Optical Amplifiers" describes a semiconductor laser which is used both for detecting and for amplifying optical signals. Coherent detection of optical signals with the aid of a semiconductor laser is described in Electronics Letters, 11th May 1989 Vol. 25 No. 10, pages 680-682, J. Mellis and M. J. Creaner: "Coherent Detection of 565 Mbit/s DPSK Data Using Semiconductor Laser Amplifier as Phase Modulator". In an article in Electronics Letters 20th Nov. 1986, Vol. 22, No. 24, pages 1307-1308, by M. S. Whalen et al entitled "Wavelength-Tunable Single-Mode Fibre Grating Reflector", there is described a fan-shaped reflection grating which can be adjusted to provide mutually different wavelengths. An optical directional coupler is described in U.S. patent specification No. 4,157,860, and the U.S. Pat. No. 4,831,631 teaches a laser diode which includes an external resonance cavity. Waveguide branches, so-called passive 3-dB couplers are known, in which two incoming waveguides are joined to a common waveguide, which rebranches into two outgoing waveguides. A light wave in one of the incoming waveguides is divided into mutually equal powers in the two outgoing waveguides. Waveguides in monocrystalline semiconductor materials are well known.
In many cases, terminals in optical communication systems include a plurality of the aforementioned components, and also other types of components. These components are discrete units which are integrated to form relatively complicated terminals. Attempts have been made to simplify these terminals, by producing integrated optical units on a common substrate. An example of one such integrated unit is disclosed in an article in Appl. Phys. Lett. 51 (20), November 1987, pages 1577-1579, H. Inoue and S. Tsuji entitled "Optical amplification by monolithically integrated distributed-feedback lasers". The article describes an arrangement including DFB-lasers, an optical switch, and amplifiers. An integrated heterodyne receiver is described in an article presented in Electronics Letters 23rd Nov. 1989, Vol. 25, No. 24, pages 1621-1623, by T. L. Koch et al and entitled "GaInAs/GaInAsP Multiple Quantum-Well Integrated Heterodyne Receiver". This receiver includes a Bragg-reflector, a waveguide, a directional switch, amplifier and detectors.
One problem with integrated units of this kind is that the various components differ greatly from one another with respect to their design, and the manufacture of the integrated unit requires a large number of separate masks in separate production stages. This renders the units comparatively expensive and production yield is relatively low, in other words the number of useful units obtained is comparatively small.
The Swedish patent application No. 8803780-9 teaches a relatively simple terminal, which can be produced on one single substrate. The terminal is able to detect, amplify and transmit a light wave. One weakness of the terminal, however, is that it cannot be used in a broadband optical communication system that uses optical signals of multiple wavelengths. In a system of this kind, several wavelengths are handled simultaneously by a terminal, for instance such that one of the wavelengths is detected and the remaining wavelengths are amplified and forwarded.